PROJECT SUMMARY/ABSTRACT Acute respiratory tract infections are a leading cause of morbidity and mortality in children worldwide. Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in children, elderly and in immunocompromised patients, as well as of asthma exacerbations. No effective treatment or vaccine for RSV is currently available, and many fundamental questions regarding the pathogenesis of RSV-induced lung disease have yet to be answered. Exosomes are microvesicles which in recent years have been shown to transfer biologically active proteins, lipids and nucleic acids, and therefore participate in cell to cell communication, inflammation, antigen presentation, programmed cell death, and disease pathogenesis. In recent investigations, we found that RSV induces significant changes in exosomal RNA cargo composition, as exosomes isolated from RSV-infected cells carry both viral proteins and viral RNA along with host mRNA and rRNA fragments, as well as small non-coding RNAs (sncRNAs), such as miRNAs, piRNAs and tRNAs, in a proportion different from exosomes isolated from mock-infected cells. Exosome-mediated export of host/viral components may have variety of outcomes, serving both as a viral strategy to evade pathogen sensing in infected cells and host strategy to induce innate responses in neighboring uninfected cells. In this application, we will investigate whether epithelial-derived exosomes modulate innate immune responses and viral replication using in vitro and in vivo models of RSV infection, and we will identify targets of novel sncRNAs enriched in epithelial-derived exosomes in response to viral infection. A better understanding of how exosomes may influence disease pathogenesis by facilitating or inhibiting immune responses in the course of RSV infection will provide valuable insights into host-pathogen interactions and possibly identify novel targets for therapy.